Anything in Network-work Wide/Events for the MX showing anything being blocked (usually by L7 firewall rules)?
Anything in Security & SD-WAN/Security/Events, show anything being blocked?
You could try getting a list of DNS entries these two are accessing. If you can't get that from the supplier easily, do a packet capture on udp/53, reboot the device and then access what you are interested in. Note the DNS queries made in the packet capture.
Then go into Security & SW-WAN into content filtering and use the tool to lookup DNS names to find what category they are in - something is probably in a blocked category.